How did roosevelt manage to give churchill the destroyers that he requested?

Roosevelt and Churchill came to an agreement called the Destroyers for Bases Agreement. The agreement stated that the US would provide what now is considered "town-class" destroyers for land rights in the British Empire. The ships were named after towns that both the countries shared in name.
The US provided 50 such ships for the land rights in Newfoundland, the Bahamas, Jamaica, St.Lucia, Trinidad, Antigua, and Guyana.

The amendment process.